A method for making glass ceramic, optoelectronic materials that contain nanocrystals that are doped with at least one kind of optically active metal.
Over the past few decades, fiber optic systems have become the standard for long-distance communication. This preponderance stems from several advantages of optical links over the more traditional metallic-based counterparts, including lower loss, higher information capacity, low cost per channel, immunity to crosstalk and electrical interference, and a smaller physical mass. Currently, optical fiber systems carry hundreds of terabits per second over distances greater than 1000 km. Even though the capacity of optical fibers is orders of magnitude beyond the capability of metallic links, the demands of global communication are driving the system capacity to double every year.
Transition metals have long been used as optically active dopants in crystalline hosts because they fluoresce in the near infrared (1000-1500 nm) region, while exhibiting a correspondingly large bandwidth. For example, disclosed in U.S. Pat. No. 4,987,575 to Alfano et al. are Cr4+ doped crystals that are capable of lasing near 1.3 xcexcm. Another example is titanium-doped sapphire (Ti:Al2O3), which provides optical gain in the range of about 650-1100 nm.
Given the useful wavelength range and bandwidth of many transition metal dopants, one can see that their advantageous attributes could be put to good use in telecommunications applications. The crystalline-host transition metal technology of U.S. Pat. No. 4,987,575, however, is not suited for these applications, since the primary optical communications medium is glass-based optical fiber. While a logical extension would be the inclusion of transition metal dopants into glasses, their performance (particularly their efficiency) has unfortunately been found to degrade in amorphous hosts, where the crystal field strength is much smaller than single-crystal hosts. The transition metal ions instead, merely are suspended in the amorphous body providing or contributing little to the amplification or transmission qualities.
Another approach has been considered by Alfano et al. in U.S. Pat. No. 5,717,517, whereby the laser-active Cr+4 (or V+3)-doped crystal is manufactured as a plurality of particles, to be dispersed in a xe2x80x9cnon-gaseousxe2x80x9d medium. In this manner, the dopants remain laser-active within a crystalline host while the larger, surrounding medium is compatible with fiber optic technology. In order to minimize the optical losses from such a composite medium, both the particles and their index difference from the surrounding medium must be small. These requirements were recognized in the patent by Alfano et al., and the particle size was therefore stipulated to be between 0.05 and 500 xcexcm, while the index mismatch was specified to be lower than 0.1.
While the concept of dispersing crystalline particles in an amorphous medium is valid, this technology has several severe drawbacks, primary of which is the manufacture of the microscopic particles and their uniform distribution in a suitable matrix. Certainly the loss decreases with particle size, and the smallest particles (0.05 xcexcm) are therefore desirable. Grinding of material generally has difficulty producing particles smaller than 1 xcexcm however, and even the sol-gel method of producing forsterite has trouble attaining particles smaller than this size. While some techniques have attained particles on the 0.5 xcexcm scale, another order of magnitude smaller seems difficult to achieve. Even allowing for the smallest particle size of 0.05 xcexcm, a simple analysis of the scattering losses reveals another major shortcoming of this technique.
To overcome the shortening of the aforementioned materials and techniques, we describe a method for making glass-ceramic optical fibers. Glass ceramics have the advantage described in a United States patent application entitled TRANSITION-METAL GLASS-CERAMIC GAIN MEDIA, filed in the name of George H. Beall, Nicholas F. Borrelli, Linda R. Pinckney, Eric J. Mozdy, on Oct. 11, 2000, which is incorporated by reference in its entirety, herein. The process of internal nucleation forms a glass ceramic, where the crystalline sites are small and uniformly distributed throughout the glass core. The crystals are formed from constituent materials of the original glass melt, not by introducing new materials as disclosed in U.S. Pat. No. 5,717,517. Moreover, the optically active dopants are introduced throughout the entire medium, as compared to only scattered particles.
When making an optical fiber from glass ceramic materials, the nature of a glass-ceramic material generally necessitates drawing the material as a glass fiber and subsequently subjecting the fiber to an appropriate thermal treatment to develop the crystalline phase. Most glass-ceramic fibers, currently known, are made by using a xe2x80x9cdouble-crucible methodxe2x80x9d. Accordingly, it has become customary to employ an apparatus known as a double crucible in drawing glasses to be converted to a glass-ceramic. The double crucible embodies a central tube for the core glass of a fiber. A larger diameter tube, surrounding the central tube, delivers the cladding glass. The respective glasses are maintained in a molten state in their crucibles, and flow from the tubular outlets to be drawn as a clad fiber.
In drawing optical fibers from glass-ceramic compositions, the most critical issue of concern is how to suppress the intense tendency of the compositions to crystallize as the glass is processed when attempting to form a glassy fiber. This phenomenon is due to the fact that the compositions for the precursor glass for a glass-ceramic, particularly the high temperature glass-ceramics useful for present purposes, are purposely designed to crystallize. Accordingly, in drawing a clad glass for present purposes, a critical problem is how to suppress this intense tendency to crystallize, thereby maintaining the fiber as a glass.
We have found various drawbacks in using the double crucible method. But the major shortcoming of this approach that the present invention is directed to ameliorate is the propensity of the respective glass components to undergo strong chemical inter-diffusion and/or interaction between the core material and the cladding material, because both glasses are in a fairly fluid or liquid state. Both the core and clad composition typically contain siginificant amounts of monovalent and divalent ions, which are likely to migrate across the core-clad interface. Diffusion problems may seriously alter the composition of the core glass-ceramic, and even render it incapable of being cerammed in a subsequent thermal treatment.
Hence, a problem exists that the present invention is directed to solving. The invention provides a method to minimize cross-diffusion between the core and cladding materials during the optical fiber manufacturing process. The method described in this application is a very different method of fiberizing a glass-ceramic material, which offers certain advantages particularly with respect to the cladding, described below, and is the preferred fiberization method for certain glass ceramic compositions.
The present invention resides in a method to produce clad optical fiber and other materials for optoelectronic applications, including lasers and amplifiers, without having to suffer unnecessarily, when forming and drawing optical fiber, contamination of the fiber core by the cladding material. More particularly, the invention provides a unique method for making an optoelectronic material by modifying the xe2x80x9crod-in-tubexe2x80x9d process to produce a clad optical fiber. Diffusion of contaminant elements into the precursor glass compositions for the glass-ceramic fiber core is kept to a minimum. Maintaining the purity of the core and its transparency to light is useful and favored in optoelectronic applications. The method can best be described as a xe2x80x9cviscous-liquid-in-tubexe2x80x9d process, wherein precursor glass compositions for making glass-ceramic materials that contain nanocrystals doped with optically active ions are employed with a more refractory or temperature resistant cladding material. According to the inventive method, a precursor glass composition is first prepared and formed into a cane. Second, a chemically inert cladding material comprising, for example, modified silica is formed into a tube that is fitted around the glass cane. Third, a glass fiber is drawn from the combined precursor-glass-cane-intube at a temperature slightly above the liquidus of the fiber-core glass composition, and subsequently at least a portion of the drawn clad glass fiber is heat treated to develop nanocrystals within the core composition, thereby forming a glass ceramic.